Thanks all for your helpful comments and thoughts.
Having managed 28 miles in the past two days I don't feel any worse for it, and the partial numbness down the front of my shinbone feels a little better, although getting on/off requires a little more thought than previously. So I'm minded to keep on with the (gentle) cycling. If nothing else, I hate being sat on a bus in traffic when I can see the cycle path and plenty of folk zipping along.
Observations from the last two days suggests that my left leg doesn't have it's usual 'power'. Not that I'm Peter Sagan, ripping up the tarmac on my ride to work as I pull a wheelie. No, it's more a case of noticing that I can't put the hammer down briefly to get away from a busy junction or stand in the pedals to get up a short, sharp hill. The power/strength/ability to command the muscles just isn't there or if feels like my leg can't manage (cue Scotty - 'The engines cannae tak ony more surr, ye cannae change the laws o' physics'). When going steady/easy it doesn't feel like a problem.
Likewise, trying to walk faster than an unconcerned amble is tricky. My leg doesn't quite seem to understand what's being asked of it. 'The leg is willing, but the flesh is weak', if you will.
The masseur I saw recently showed me some very gentle stretches to give me some relief just now, but looking ahead I'm minded to do some more serious work on core body strength to try and avoid further problems. Although this episode hasn't been as bad as one three years ago (when Mandopicker did uncanny impersonations of an australopithecine) I'd rather not go through this again...
As I've been using Ibuprofen for a couple of weeks, I'll go see my GP next week for advice on what to do. I daresay I'll be told it's 7,8,9 weeks wait to see a physio/other specialist and if I can pay privately, I should do so (this is what I was told last time round).